1. Field of the Invention
The present invention relates to a cleaning member configured to remove a contaminant adhered onto a roller surface, a charging device equipped with the cleaning member, a process cartridge, and an image forming apparatus.
2. Description of the Related Art
In an electrophotographic image forming apparatus, a charging roller is widely used as a charging unit configured to apply electric charge to a surface of a latent electrostatic image bearing member (hereinafter, also referred to as a photoconductor, image bearing member, or latent image bearing member). The charging roller is used in a contact system in which an elastic roller is contacted with a photoconductor, or a non-contact system in which gap formation members are provided on both ends of a roller having rigidity or high hardness so as to be apart from the charging roller from a photoconductor i.e., have a minute gap therebetween.
A photoconductor is charged by a charging roller, and a latent image is formed on a surface of the photoconductor by optically writing an image thereon. Thereafter, after developing and transferring steps, a residual toner is generally removed by a cleaning unit for photoconductor. As the cleaning unit for photoconductor, a system in which a photoconductor is cleaned only with a blade such as a polyurethane rubber, a system in which a photoconductor surface is supplied with a lubricant in order to improve cleanability, and the like have been known.
The charging roller is contaminated by a small amount of a toner and lubricant passing through the cleaning unit for photoconductor, and its chargeability is decreased over time. For example, when contamination progresses only in a part of the charging roller in an axial direction thereof, the resistance is increased only in the contaminated part, and then the photoconductor is not charged, thereby black streaks occur in an output image.
In order to prevent the charging roller from contamination, it is known a charging-roller cleaning device configured to clean a surface of the charging roller by bringing the charging roller into contact with a roller-shaped member formed of a fiber, such as a brush roller, or an elastic body such as a foam; for example, a cleaning member using a foam containing a melamine compound is proposed in Japanese Patent Application Laid-Open (JP-A) Nos. 2003-66807, 2005-70750 and 2006-113199.
JP-A No. 2003-66807 discloses a foam having a three-dimensional network structure used as a cleaning member, wherein a melamine resin having a fibrous form in a submicron order enters into minute convexoconcaves on an image bearing member or charging member so as to scrape out contamination, and a surface of a cleaned body can be always kept free from contamination.
However, when the linear velocity of the charging roller to be cleaned and that of the cleaning member differ in a contact portion, the contaminant may be imprinted on a surface of the charging roller. In order to prevent a difference in linear velocity between the charging roller and the cleaning member, they may be connected with a gear. However, as both of them substantially vary in outer diameter (component tolerance), the difference in linear velocity therebetween cannot be completely eliminated. Therefore, a contaminant may be imprinted on the surface of the charging roller.
In image forming apparatuses such as copiers, facsimiles, or printers, a latent electrostatic image corresponding to image information is formed on a latent electrostatic image bearing member, i.e. a photoconductor, which has been uniformly charged by a charging device, and processed to be a visible image using a developer such as a toner, and then the visible image is transferred onto and fixed on a recording medium, such as a sheet, thereby obtaining an output image.
Conventionally, as charging devices used in electrophotographic image forming apparatuses such as copiers or printers, non-contact charging devices using corona discharge have been often used. However, corona products such as ozone, nitrogen oxide and the like are generated by the corona discharge, causing environmental contamination. Moreover a nitric acid compound (ammonium nitrate) produced from a nitrogen oxide adheres to the charging devices, adversely affecting the chargeability.
On the other hand, contact charging devices having a charging roller, of which surface is directly contacted with or placed with a minute gap close to a latent image bearing member, are most commonly used in recent years, because generation of ozone and nitrogen oxides can be considerably reduced.
A charging roller is used, in which an elastic body whose resistance is adjusted, is provided around the circumferential surface of a cored bar, so as to be placed in contact with or placed with a minute gap close to a latent image bearing member, and a charge bias is applied to the charging roller. As for the charge bias, a direct-current voltage, and an alternating-current voltage having an inter-peak voltage whose voltage is twice or greater the discharge starting voltage of the direct current voltage are used, and superimposingly applied to the charging roller. By application of the alternating-current voltage, the potential of the latent image bearing member can converge to a potential of the direct current voltage applied thereto. As a result, the latent image bearing member surface can be uniformly charged.
However, in such contact charging device, the charging roller is arranged so as to face the latent image bearing member surface which has been subjected to a cleaning step. However, a toner or paper powder remaining on the latent image bearing member surface after the cleaning step tends to adhere onto the charging roller. When the charging roller is partly contaminated, the surface resistance is changed. As a result, the latent image bearing member cannot be uniformly charged.
Consequently, the following techniques of cleaning rollers have been proposed.
For example, JP-A Nos. 2007-171381 and 2007-140167 propose a technique in which an elastic cleaning roller is bitten into a charging roller so as to be rotated according to the rotation of the charging roller, thereby preventing the charging roller from contamination.
The technique has an advantage that a distance between the axis of the charging roller and the axis of the cleaning roller is adjustable, the linear pressure (nip width) can be set constant, and is free from influence of a variation of the roller diameter and change over time. Moreover, a driving unit is not necessary, so that the cost and size of an image forming apparatus can be downsized. On the other hand, the technique has a disadvantage that the drive transmission is not kept stable and that the charging roller slips because the cleaning roller is driven by a frictional force between the charging roller and the cleaning roller.
Moreover, Japanese Patent (JP-B) No. 3356603 and JP-A No. 7-140763 propose a technique in which an elastic cleaning roller is bitten into a charging roller and driven using a driving unit, so as to prevent the charging roller from defacement caused by a difference in linear velocity between the charging roller and the cleaning roller.
The technique enables to adjust a linear velocity because the cleaning roller is driven by the driving unit. However, there is a disadvantage that a linear pressure (nip width) varies depending on a variation of the roller diameter and change over time, because the distance between the axis of the charging roller and the axis of the cleaning roller is necessary to be fixed. Moreover, the number of components is increased, and the size of an apparatus becomes larger, compared to the technique of rotation according to the rotation of another member.
It has been known that, in the contact charging device in which an alternating-current voltage is superimposed, the latent image bearing member surface is just like being constantly etched by the charging utilizing a pulse discharge generated in a minute gap provided between a latent image bearing member surface and a charging roller.
When the latent image bearing member surface is abraded due to the pulse discharge of the charging roller, the thickness of a photosensitive layer on the latent image bearing member surface is decreased, i.e. film thickness loss is caused, adversely affecting the chargeability. As a result, image quality may degrade.
In recent years, the film thickness loss of the latent image bearing member is decreased by a method in which the latent image bearing member surface is applied with a solid lubricant such as zinc stearate, so as to form a lubricating layer thereon, thereby enhancing the abrasion resistance.
The film thickness loss of the latent image bearing member is significantly decreased by application of the solid lubricant. However, a part of a toner and paper powder, which have adhered onto the latent image bearing member surface with low frictional properties, tends to pass through a cleaning position along with the lubricant.
The adhesion of the lubricant as well as the toner and paper powder onto the charging roller causes a difference in circumferential velocity between the charging roller and the cleaning roller (JP-B No. 3356603 and JP-A No. 7-140763), and an inappropriate linear pressure of the cleaning roller (JP-A Nos. 2007-171381 and 2007-140167). In these cases, filming occurs on the charging roller, inhibiting the longer operating life of the charging roller.